Voltage controlled oscillators utilizing piezoelectric elements have been widely used as frequency signal sources in various communications devices and other electronic devices.
Moreover, voltage controlled oscillators utilizing, as a load capacitance of the piezoelectric element, at least one variable capacitive element whose capacitance value varies according to a DC control voltage have been used to suppress frequency differences over a wide range of usage temperatures, to synchronize a frequency to a reference frequency, and the like.
In this type of voltage controlled oscillator, a wide range of frequency variation and linearity of oscillation frequency with respect to control voltage are desirable. Specifically, in order to improve the linearity of oscillation frequency with respect to control voltage, it is necessary to ensure that the load capacitance is linear with respect to the control voltage.
The devices described in patent document 1 and patent document 2 are known voltage controlled oscillators of this type.
As shown in FIG. 17, the voltage controlled oscillator described in patent document 1 includes a CMOS inverter 1, a quartz oscillator 2 connected in parallel between the input and output terminals of the CMOS inverter 1 to form a feedback loop, a resistor 3 forming a feedback loop, fixed capacitors 4 and 5 respectively connected to input and output sides of the CMOS inverter 1, a variable capacitance element 6 with a capacitance value that changes according to an applied control voltage Vc and is connected in series with the fixed capacitor 4 on the input side of the CMOS inverter 1, and a bias-use resistor 7.
In this type of voltage controlled oscillator the oscillation frequency generated using the quartz oscillator 2 is varied by causing the capacitance value of the variable capacitive element 6 connected on the input side of the CMOS inverter 1 to vary using the control voltage Vc.
As shown in FIG. 18, the voltage controlled oscillator described in patent document 2 includes an amplifier circuit 11, a piezoelectric element 12 that is connected in parallel between an input terminal and an output terminal of the amplifier circuit 11 and forms a feedback loop, a resistor 13 that forms a feedback loop, variable capacitive elements (varicap) 14 and 15 respectively connected to the input and output sides of the amplifier circuit 11 and each having a capacitance value that changes according to an applied control voltage Vc, and a frequency-adjustment voltage generating circuit 16 that generates the control voltage Vc.
In the voltage controlled oscillator with this type of construction, the oscillation frequency generated using the piezoelectric element 12 is varied by changing the capacitance values of the variable capacitive elements 14 and 15 that are connected to respective terminals of the piezoelectric element 12. The control voltage Vc is generated by the frequency-adjustment voltage generating circuit 16, using as the load capacitance the variable capacitive elements 14 and 15 with the capacitance value that changes according to the control voltage Vc.
[Patent Document 1] JP2003-282724A
[Patent Document 2] JP10-51238A
As shown in FIGS. 17 and 18, forms of the voltage controlled oscillator having at least one variable capacitive element include a form in which a single variable capacitive element connects to a terminal on one of the input and output sides of the amplifier circuit and a fixed capacitor connects to the other terminal, and a form in which a variable capacitive element is connected each of the two terminals of the amplifier circuit.
For both forms, a load capacitance CL that determines the oscillation frequency is a series capacitance of an input-side capacitance Cin and an output-side capacitance Cout. The series capacitance is expressed below in equation (1).CL=(Cin×Cout)/(Cin+Cout)  (1)
The following is a discussion of a variable range of the oscillation frequency for the voltage controlled oscillators shown in FIG. 17 and FIG. 18.
As shown in FIG. 17, in the form in which the variable capacitive element connects to the terminal on one of the input and output sides of the amplifier circuit, the above-described load capacitance is a combined capacitance that includes the capacitance of the variable capacitive element connected to one of the input-terminal or the output-side terminal of the amplifier circuit and the capacitance of fixed capacitor connected to the other terminal.
As shown in FIG. 18, in the form in which a variable capacitive element connects to both terminals of the amplifier circuit, the above-described load capacitance is a combined capacitance that includes the capacitance of the variable capacitive element connected to the input-side terminal and the capacitance of the variable capacitive element connected to the output-side terminal.
Thus, from equation (1) it is clear that the amount of change in load capacitance will be greater in the latter form.
Therefore, in the form, shown in FIG. 18, in which variable capacitive elements are attached to both the input and output-side terminals of the amplifier circuit, an achievable oscillation frequency range is wider than in the form, shown in FIG. 17, in which the variable capacitive element is used only on one side.
The following is a discussion of the linearity of the change in oscillation frequency with respect to control voltage in the voltage controlled oscillators shown in FIG. 17 and FIG. 18.
As described above, the oscillation frequency is determined by the load capacitance. Hence, to have linearity in the change in oscillation frequency with respect to control voltage, it is necessary to have linearity in the change in load capacitance with respect to control voltage.
FIG. 19 shows an example of the changes in input and output-side capacitances with respect to control voltage for the form in which the variable capacitive element connects, as shown in FIG. 17, to one of the input-side and output-side terminals of the amplifier circuit.
The capacitance of the variable capacitive element changes according to the control voltage, but the fixed capacitance is constant and independent of the control voltage. The change in the load capacitance, which is the combined capacitance calculated using equation (1), with respect to the control voltage is shown in FIG. 20. As is clear from FIG. 20, in a region where the change in capacitance begins, the change in the load capacitance is large, but in a region where the change in capacitance ends, the change in load capacitance becomes smaller. In short, the change in load capacitance with respect to control voltage is non-linear.
In the other form, in which variable capacitive elements are used at both the input and output terminal of the amplifier circuit as shown in FIG. 18, a central operating voltage and an amplitude of a oscillated wave form differ at the input-side terminal and output-side terminal of the amplifier circuit. Hence, the change in capacitance of the variable capacitive element with respect to the control voltage is different on the input and output sides.
Thus, an input voltage range over which the change in capacitance occurs in the variable capacitive element connected to the terminal where the amplitude of the oscillation wave form is large, is larger than a section over which the change in capacitance occurs in the variable capacitive element connected to the terminal where the amplitude of the oscillation wave form is small. A smallest value and a largest value of the oscillation wave form vary according to differences in the central operating voltage and the amplitude of the oscillation wave form. Hence, the control voltage at which the change in capacitance begins and the control voltage at which the change in capacitance ends differ between sides.
Generally, in an oscillator of the type shown in FIG. 18, the oscillation wave form on the output side has a higher central operating voltage than the wave form on the input side. Also, the change in the capacitance of the variable capacitive elements on the input and output sides with respect to the control voltage when the amplitude is large, is as illustrated in FIG. 21. The change in the capacitance begins at a higher control voltage in the output-side variable capacitive element than in the input-side variable capacitive element. Moreover, the range of control voltage over which the change in the capacitance occurs is larger.
In this case linearity of the change in the load capacitance with respect to the control voltage is poor, as shown in FIG. 22.
Thus, in conventional voltage controlled oscillators that use variable capacitive elements, obtaining a wide frequency variation range and making the change of oscillation frequency linear with respect to the control voltage are problems.
The object of the present invention is solve these problems by providing a voltage controlled oscillator having a wide frequency variation range and an oscillation frequency that shows favorable linearity with respect to control voltage.